Patent Application
20180306534
This disclosure relates generally to heat exchangers. More particularly, this disclosure relates to improved structures and geometries for glass heat exchangers, which are more efficient in gas heat exchange.
Heat exchangers are devices that facilitate the transfer of heat between mediums. Such devices are found in a large number of applications, ranging from air-conditioning units, to engines, and so on. In some heat exchangers, efficiency is determined by the effectiveness of the heat exchanger in thermally isolating opposing sides of the heat exchanger such that a gas or other working fluid flowing therebetween transfers heat to the heat exchanger between a hot end and a cold end of the heat exchanger. One particular application of a heat exchanger where such an efficient heat gradient is of particular importance is in a cryogenic cooler (“cryocooler”), which may utilize the cold end to effectively cool various components, such as electronics, superconducting magnets, optical systems, or so on.
The primary use of the heat exchanger in systems such as cryocoolers may be to pre-cool the working gas as it is transferred from the hot end to the cold end of the machine. Such heat exchangers may be characterized by how the gas flows through the exchanger and the surrounding system. For example, many closed cycle, linear cryocooler systems utilize the Stirling cycle, wherein a working gas cyclically flows in opposing directions through the heat exchanger. Such systems are typically referred to as regenerative heat exchangers, or regenerators. In other systems, a working gas steadily flows through the heat exchanger, utilizing processes such as the Joule-Thompson effect to create the cold end. The heat exchangers of these steady flow systems are typically referred to as recuperative heat exchangers, or recuperators.
The effectiveness of heat exchangers may be dependant upon various factors, such as heat transfer effectiveness, pressure drop, heat capacity, and parasitic conduction of heat. In regenerative systems, the gas is compressed at the hot end of the regenerator, and will be allowed to expand after it reaches the cold end. The structure of the heat exchanger itself may prevent the transfer of significant amounts of heat to the cold end as it flows. In regenerative systems, the oscillating rate of gas flow is typically of a high frequency. Therefore, the rate of heat transfer from the working gas to the regenerator should be rapid to ensure a desirable amount of pre-cooling of the gas through the heat exchanger.
Minimizing pressure drop across the heat exchanger is also desirable in increasing cooler efficiency, however this is typically at odds with maximizing the rate of heat transfer because obtaining maximum heat transfer effectiveness is generally through maximizing the mount of solid surface area over or around which the gas flows, which may create flow friction for the gas, and thus increase the pressure drop. In many heat exchangers, the cross-sectional flow area and parameters of porosity for the heat exchanger are varied to balance minimal pressure drop and maximum heat transfer.
The heat capacity of the heat exchanger must be such that the exchanger may absorb heat from the working gas without experiencing an intrinsic temperature increase which may reduce system efficiency. An interplay between the specific heat of the heat exchanger materials and the specific heat of the working gas exists, and may be particularly troublesome when cryogenic temperatures are sought to be achieved at the cold end of the exchanger. As one example, the specific heat of helium (a common working gas) is relatively high at cryogenic temperatures, while the specific heat of common heat exchanger materials is lower at cryogenic temperatures than at room temperature. This may call for an increased volume or mass for the heat exchanger.
The material selection for the heat exchanger is also important in preventing parasitic conduction of heat, for example along the axis of the heat exchanger. Where a large temperature gradient occurs along the length of the heat exchanger, it is very desirable that the exchanger have low thermal conductivity along its length, as high conductivity may result in heat being conducted from the hot end to the cold end. This conducted heat is a parasitic reduction of efficiency, because it must be carried as part of the refrigeration that is produced by the cycle.
One type of conventional heat exchanger typically contains a large number of woven-wire screens (i.e. on the order of 1000 screens in some embodiments) that are packed together into a volume. The working gas flows through the screens of the volume, so that the screens, which are typically formed from stainless steel, absorb the heat from the gas. The screen material may be similar to that of typical filter screens, with hundreds of wires per inch of material and wire diameters on the scale of a thousandth of an inch. The wires are generally drawn from stainless steel stock, a material that exhibits acceptable heat capacity and thermal conductivity.
There are limitations to stacked screen heat exchangers, however. For example, the heat capacity of the stainless material drops to unacceptably low levels at low cryogenic temperatures (i.e. below 30K). Additionally, construction limitations on the screens permit only a relatively small range of regenerator porosities, the ratio of regenerator open volume to overall regenerator volume (typically 60-75%). Similarly, the pore size between rows of wire is limited. Restrictions on achievable porosity and pore size limit the ability of a cryocooler designer to effectively optimize the relationship between pressure drop, heat transfer effectiveness and heat capacity. As an example, at very low temperatures, such as those encountered in the 2nd stage of a multi-stage cryocooler, the ideal screen regenerator might have a porosity significantly lower than 60% such that the solid volume (and hence heat capacity) is increased in order to combat the reduction in specific heat of the stainless steel at such low temperatures. However, porosities significantly below 60% are difficult to obtain using stainless steel screen technology.
Another type of conventional heat exchangers contains packed sphere beds. The working glass flows through the spaces between the spheres of the exchanger, transferring heat into the spheres as it moves through the heat exchanger. The sphere bed heat exchangers have an advantage of being able to utilize materials that may not easily be formed into woven screens, such as lead or rare-earth metals, that may exhibit high specific heats at low cryogenic temperatures. Sphere bed heat exchangers also have an additional benefit of permitting a lower porosity for the heat exchanger (i.e. below 40% for some embodiments), which can be achieved due to the inherent geometry of the sphere pack. The lower porosity allows more solid material, and thus greater heat capacity, while maintaining an acceptable tolerance of pressure drop for many applications. In some cryocoolers utilizing packed spheres, temperatures as low as 11K at the cold end have been achieved. Despite this success, sphere beds are less effective at higher temperatures, where heat capacity is less of a concern than pressure drop.
A more recent development in heat exchanger technology has been the use of glass as the heat exchanging element. Glass manufacturing processes include etching, grinding, or machining, which may permit, among other things, greater degrees of shaping and control of the porosity of the heat exchanger. The present manufacturing of heat exchangers typically involves etching or scoring panes of glass, which are then bonded together to form heat exchange elements. Among other things, the bonding process, or the presence of the bond between the glass layers, may reduce the effectiveness of the glass in exchanging heat with the gas flowing through the etched layers. In other cases, heat exchangers may be formed by a plurality of perforated glass plates, having slots etched in each layer, separated by spacers.
What is needed is, among other things, improvements over known heat exchanger geometries and structures, which permit a more effective heat transfer without resulting in an excessive pressure drop.
According to an embodiment, a heat exchanger may comprise a glass body having a first flat face and a second flat face on opposing ends. The first flat face and the second flat face may define a longitudinal axis therebetween. The heat exchanger may further have a plurality of holes in the glass body. The holes may be elongated along the longitudinal axis by extending from said first flat face to said second flat face. The plurality of holes may be configured to receive and direct a gas therethrough to exchange heat between the gas and the glass body.
Other aspects and embodiments will become apparent from the following detailed description, the accompanying drawings, and the appended claims.
Various features of embodiments of this disclosure are shown in the drawings, in which like reference numerals designate like elements.
Glass body 20 may be of any appropriate shape. In the illustrated embodiment, glass body 20 has a generally annular cross sectional configuration around central aperture 30. In other embodiments, glass body 20 may lack central aperture 30, and may be of a circular or elliptical cross sectional configuration, such that glass body 20 approximates a cylinder. In further embodiments, glass body 20 may be of any other appropriate geometric shape, including having a triangular, rectangular, pentagon, hexagon, U shaped, or any other multi-sided cross section (forming a geometric prism or other polyhedron). In various embodiments central aperture 30 may be formed in or around these alternative shapes. Furthermore, central aperture 30 may be of any shape or configuration, including defining a space having any cross section, including those described above for glass body 20.
Central aperture 30 may be configured for any suitable purpose. For example where heat exchanger 10 is configured to be used in a cryocooler, central aperture 30 may be configured to couple with a portion of the cryocooler. In an embodiment, the cryocooler may comprise a portion extending therefrom, such as a pulse tube, which may be received by central aperture 30 to connect heat exchanger 10 into the cryocooler. In other embodiments, central aperture 30 may be configured to receive other elements. For example, in embodiments in which heat exchanger 10 is being used in a heat engine, central aperture 30 may be configured to receive a moving piston for the heat engine.
First flat face 40 and second flat face 50 are spaced on opposing ends of glass body 20. In the illustrated embodiment, first flat face 40 and second flat face 50 are configured in approximately parallel planes. As shown, first flat face 40 and second flat face 50 are depicted as equivalent to any given cross section of glass body 20, because of this uniformity. In other embodiments, first flat face 40 and second flat face 50 may be intentionally angled with respect to one another, or with respect to other portions of glass body 20.
The formation of glass bodies 20 with holes 70 may be by any suitable process. In an embodiment, holes 70 may be formed from drawn-glass flow tubes. In some embodiments, holes 70 may be etched from glass body 20 by exposure to a chemical rinse. In an embodiment, fibers of etchable core glass surrounded by non etchable cladding glass are stacked into hexagonal close-pack multifiber, which may be drawn to fuse the fibers together. In an embodiment, the hexagonal close-pack multifibers may then be stacked into a large array, and fused under pressure, which may reduce or eliminate interstitial voids. In an embodiment, the etchable core glass of each individual fiber may support the channels. In an embodiment, the fused body may be cut and ground into a blank for glass body 20, from which glass bodies 20 may be cut. In an embodiment glass body 20 may be subsequently placed in an etching solution to remove the soluble components, leaving voids that are holes 70.
As noted above, the plurality of holes 70 may be configured to receive and direct a gas therethrough, so as to exchange heat between the gas and glass body 20. In essence, glass body 20 of heat exchanger 10 may act as a gas-solid heat exchanger. In various embodiments, the size, shape, and number of holes 70 in glass body 20 may be selected to tune the porosity of glass body 20, to affect the flow of gas through heat exchanger 10. For example, holes 70 may sized and shaped to optimize surface area against which the gas may contact to transfer heat to glass body 20. As the gas flows along the plurality of holes 70 from first flat face 40 to second flat face 50, or vice versa, hot gas may transfer that heat to glass body 20, while cool gas may receive heat from glass body 20. Additionally, having a straight channel from first flat face 40 to second flat face 50 may reduce collisions of gas molecules, resulting in an reduced pressure drop between first flat face 40 and second flat face 50. In an embodiment, the size of holes 70 across first flat face 40 and/or second flat face 50 may be selected based on the amount of gas flowing through heat exchanger 10. In an embodiment, a higher capacity system may have a greater mass of gas flowing therethrough, so a larger width of holes 70 may reduce the gas velocity. In an embodiment, the width of holes 70 may be optimized based on the operating point, type, and/or cooling capacity of the system containing heat exchanger 10.
The material selection for glass body 20 may ensure thermal isolation between portions of glass body 20 closer to first flat face 40 and portions of glass body 20 closer to flat face 40. In an embodiment, each glass body 20 may be configured to thermally isolate first flat face 40 and second flat face 50 at a temperature differential of approximately 10-50K. In other embodiments, wherein glass body 20 is longer, a greater temperature differential may be achieved. In an embodiment, each of plurality of holes 70 of glass body 20 may be substantially the same size across first flat face 40 and/or second flat face 50, so as to increase consistency of gas flow through glass body 20, thus reducing or preventing differential or preferential flow. As noted above, in an embodiment, heat exchanger 10 may be assembled into a system, such as a cryocooler or a heat engine. In such embodiments, flat face 40 and second flat face 50 of heat exchanger 10 may be aligned along the flow path of a gas that flows through heat exchanger 10 that is used in the system.
In some embodiments of heat exchanger 10, such as those shown in the perspective and top views of
In an embodiment, glass body 20 may have portions of holes 70 surrounding exterior sides 60. Such portions of holes 70 may result from cutting and/or shaping glass body 70 from glass that already has holes 70 formed therein. In an embodiment, exterior housing 80 may permit gas to flow between the exterior sides 60 of glass body 20 and interior sides 90 of exterior housing 80, in particular through partially formed holes 70. As noted above, however, having same sized holes 70 is preferred in glass body 20 to prevent differential flow, so partially formed holes 70 at the exterior sides 60 of glass body 20 may be undesired. In an embodiment, an area around first flat face 40 and/or second flat face 50 of glass body 20 may be covered by caps to prevent gas flow through partially formed holes 70. In an embodiment, glass body 20 may be secured into exterior housing 80 so as to seal partially formed holes 70. In an embodiment, glass body 20 may be secured by glue or epoxy into exterior housing 80, which may fill in partially formed holes 70.
In an alternative embodiment shown in
In an embodiment, exterior housing 80 may be configured such that gas flowing through each of glass bodies 20 does not leak out between adjacent glass bodies 20. In some embodiments, stacks of glass bodies 20 may be utilized to overcome limits in formation of holes 70 in each glass body 20. For example, in some embodiments in which holes 70 are etched into each glass body 20 by a chemical bath, the etchant may be unable to traverse glass body 20 if glass body 20 is greater than a certain length. In some cases, holes 70 may then not be consistently etched from first flat face 40 to second flat face 50, leaving holes 70 that are partially or completely blocked off within glass body 20.
In some embodiments, holes 70 in adjacent glass bodies 20 may be aligned such that gas flowing through hole 70 in a first one of glass bodies 20 may substantially or completely enter an associated hole 70′ in a second one of glass bodies 20′. Such alignment may be accomplished by any suitable mechanism, including but not limited to laser-based alignment. Due to variability in manufacturing of glass bodies 20, however, such alignment may be difficult, or unnecessary. In some embodiments, holes 70 in one glass body 20 may generally at least partially overlap two or more associated holes 70′ of an adjacent glass body 20′, such that, for example, gas traverses through the first hole 70, before splitting into two or more holes 70′ of the adjacent glass body 20′. In an embodiment, holes 70 may be configured such that random orientation of glass bodies 20 may permit sufficient movement of gas between adjacent glass bodies 20 with minimal pressure drop. For example, in an embodiment, the arrangement of holes 70 in a glass body 20 may be such that the size of the holes 70 are larger than the connecting portions of glass body 20, permitting ease of gas flow transitions between glass bodies 20. As the number of transitions in the heat exchanger 10′ are smaller than those between the stacked metal screens of conventional heat exchangers, friction from gas flow may still be reduced as compared to conventional exchangers by this improved configuration.
In
As noted above, heat exchanger 10 may be utilized in any number of applications, including but not limited to a cryocooler or a heat-engine. The direction of flow for the gas through heat exchanger 10 may change depending on the specific application. For example, some cryocoolers may make use of a liner closed-cycle configuration, such as the Stirling cycle in which gas oscillates back and forth through heat exchanger 10. As another example, some heat engines utilize the Stirling cycle, heating the gas on one side of heat exchanger 10 and cooling the gas on the other, such that movement from the expansion and contraction of gas therethrough generates electrical or mechanical energy which may be harnessed.
In embodiments wherein the working gas oscillates through heat exchanger 10, heat exchanger 10 may be characterized as a regenerator. In an embodiment, this oscillation may be at a rate of approximately 20-100 Hz. In an embodiment, as gas flows from a hot end of the cryocooler through heat exchanger 10 to a cold end of the cryocooler, the gas may give up heat to glass bodies 20 in heat exchanger 10. As the flow reverses to flow from the cold end to the hot end, the gas may absorb heat back from glass bodies 20. Because of this cyclic pattern, the net energy gain in heat exchanger 10 over any cycle when in this configuration may be approximately zero.
In other embodiments, the working gas may be configured to flow in one direction through glass bodies 20 of heat exchanger 10. In such steady flow embodiments, which may operate by any number of mechanisms, including but not limited to the Joule-Thompson effect. As an example, gas may flow through heat exchanger 10, and be cooled as it flows through holes 70 of glass bodies 20, which act as the valve for the throttling process. In other embodiments, the length of glass bodies 20 may merely be configured to act as a solid-gas heat exchanger, such that as the gas flows through holes 70, heat transfers to glass bodies 20, and radiates outward from glass bodies 20 to the ambient environment. In an embodiment heat exchanger 10 configured to operate in a steady-flow embodiment may be characterized as a recuperator.
Regardless of the presence of a reversal of the direction of gas flow, in various embodiments as the gas flows axially through the plurality of holes 70, the gas may cool from first flat face 40 to second flat face 50. In an embodiment, the number of glass bodies 20 in heat exchanger 10 may be selected based on the amount of cooling and thermal separation required between the hot end and the cold end of heat exchanger 10. In an embodiment, a set of approximately 5 to 10 of glass bodies 20 may be assembled into heat exchanger 10. In an embodiment, heat exchanger 10 may be configured to thermally isolate the hot end and the cold end to prevent the parasitic conduction of heat from the hot end to the cold end. In an embodiment, the temperature differential between the hot end and the cold end of heat exchanger 10 may be approximately 200K. For example, the temperature may be approximately 100K at the cold end of heat exchanger 10 and approximately 300K at the hot end of heat exchanger 10, to achieve cryogenic cooling in an approximately room temperature environment. In some embodiments, such as where the system utilizing heat exchanger 10 operates in cryogenic temperatures, the cold end of heat exchanger 10 may be any cryogenic temperature (i.e. typically below 125K). In an embodiment, to achieve low cryogenic temperatures, glass bodies 20 may be configured to have a lower porosity (such as by tuning the size and number of holes 70) to achieve a lower pressure drop.
While certain embodiments have been shown and described, it is evident that variations and modifications are possible that are within the spirit and scope of the inventive concepts as represented by the following claims. The disclosed embodiments have been provided solely to illustrate the principles of the inventive concepts and should not be considered limiting in any way.
This application is a continuation of U.S. patent application Ser. No. 12/888,306 filed on Sep. 22, 2010.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12888306 | Sep 2010 | US |
| Child | 16019200 | US |
